This invention relates to scanning devices employing a rotatable mirror device such as those used in laser printing devices and specifically to facet tracking devices. FIG. 1 shows a typical laser printing device utilizing a rotating mirror for scanning. A laser 10 emits a beam 12 towards pre-polygon optics 14. After passing through the pre-polygon optics 14, the beam 12 falls on a scanner 16. The scanner 16 is a rotating polygon with flat reflecting facets 18. As the scanner 16 rotates, the beam 12 is scanned along a scan line on a photoreceptor 22. The direction along the scan line 20 is the tangential plane and the direction perpendicular to the scan line on the photoreceptor 22 is the sagittal plane.
Scanner performance is determined by the physical limitations on the speed at which the mirror is rotated, by the angular deflection of the laser beam achieved by reflection from a facet from the rotating polygon, the number of facets, the size of the facets, and the width of the beam being scanned where it is incident on the rotating mirror.
The beam width impacts the scanning speed because it determines the minimum facet size of a facet on the rotating mirror. A larger facet means a larger rotating polygon and hence larger, more costly motor polygon assemblies with higher power motors and slower scanning speeds. Scanning speeds, for a given beam width, can be increased by the use of facet tracking devices because they allow a smaller facet to be used and therefore smaller rotating mirrors which can be rotated faster.
One method for increasing scanning speeds is the use of angle doubling with small sized polygon assemblies having a large number of small sized facets. For an "F-THETA" scan lens, commonly employed in laser scanners, the scanned distance on the photoreceptor is the product of the scan angel (THETA) and the effective focal length (F). Whenever the scan angle can be increased, the effective focal length can be decreased for a given scan length. A decrease in effective focal length brings two primary advantages. Firstly, the smaller focal length translates directly into a smaller physical casting or base upon which the optical components are mounted. Glass lens elements, mirrors and all other components can be smaller. The end result is a smaller, lighter, less costly product. Secondarily, the shorter focal length requires a smaller beam at the rotating polygon, further reducing the sizes of optical and mechanical components.
A further advantage results from scan angle doubling in that any given scan distance along the photoreceptor can be achieved with only half the polygon angular rotation. By this means, the polygon speed of rotation is significantly reduced, allowing lighter, smaller and less costly motor bearings as well as better bearing lifetime and overall performance.
Facet tracking and angle doubling devices are known and have been described as in U.S. Pat. No. 3,973,826 by Lobb which describes a device for passive facet tracking and angle doubling. Lobb describes a system utilizing a prescanner which, as it rotates with the scanner, produces a variable deflection in the scanned beam so that during a scan period, the beam moves at the speed of a scanner and in the same direction. The prescanner, by slightly deflecting the beam at the speed of the scanner and in the same direction, maintains the position of the beam centered in the scanning facet. Specifically, a beam, which is focused on a prescanner, is reflected off the prescanner to a concave mirror which causes the beam to converge but not focus, on a facet of the scanner. The prescanner is built using cylindrical or curved facets and the scanner is built using flat facets.
Lobb also describes two scan angle doubling configurations. The first comprises a rotating mirror which reflects light into a static optical system. The static optical system reflects the received light back onto the rotating mirror. The static optical system is comprised of a single system consisting of a roof prism and a field lens or a plurality of static optical systems arranged in an arc in the scanning area, each system comprising a roof prism and a field lens.
In the Lobb patent, the beam is not collimated at the scanner facet in the scanning plane, thus any variation in radius between the facets will translate into scanning errors on the scanning plane. In a laser printing application, these scanning errors show up as pixel placement errors visible on a printed page. When the scanned beam is collimated in the scanned plane at the scanner facet, polygon manufacturing tolerances can be relaxed with resultant cost savings.
In the Lobb patent, the beam is not focused on the scanner facet in the sagittal plane. As a consequence, pyramidal errors in the scanner facet and bearing wobble can not be compensated but will result in variable spacing between scan lines. In a laser printing application, these errors show up on the printed page as differences in spacing between the printed lines. Even very small differences are apparent, producing unacceptable output quality. When the beam is focused on the scanner facet in the sagittal plane, pyramidal errors may be optically removed by focussing both the beam and the facet onto the scan line in the sagittal plane. Again, this allows polygon manufacturing tolerances to be relaxed with resultant cost savings and no loss in print quality.
The present invention uses anamorphic optics to collimate the beam in the tangential plane at the scanning facet and to focus the beam in the sagittal plane at the scanning facet so that errors produced by radial and pyramidal variations of the scanning facets may be substantially reduced or easily corrected to provide for improved scanning.
Further advantages of the invention will become apparent as the following description proceeds.